Charged particle beam systems are used in a variety of applications, including the manufacturing, repair, and inspection of micro-fabricated devices, such as integrated circuits, magnetic recording heads, and photolithography masks. Dual beam systems, such as the DualBeam instruments commercially available from FEI Company, the assignee of the present invention, typically include a scanning electron microscope (SEM) that can provide a high-resolution image with minimal damage to the target, and an ion beam system, such as a focused or shaped beam system (FIB), that can be used to alter substrates and to form images. Such deal beams systems are described, for example, in U.S. Pat. No. 7,161,159 to Hill, et al., which is incorporated by reference in its entirety in the present application. In some dual beam systems, the FIB is oriented an angle, such as 52 degrees, from the vertical and an electron beam column is oriented vertically. In other systems, the electron beam column is tilted and the FIB is oriented vertically or also tilted. The stage on which the sample is mounted can typically be tilted, in some systems up to about 60 degrees.
A common application for a dual beam system is analyzing defects and other failures during micro-fabrication to troubleshoot, adjust, and improve micro-fabrication processes. Defect analysis is useful in all aspects of semiconductor production including design verification diagnostics, production diagnostics, as well as other aspects of microcircuit research and development. As device geometries continue to shrink and new materials are introduced, the structural complexity of today's semiconductors grows exponentially. Many of the structures created with these new materials are re-entrant, penetrating back through previous layers. Thus, the defects and structural causes of device failure are often hidden well below the surface.
Accordingly, defect analysis often requires cross-sectioning and viewing defects on a three-dimensional basis. With the growing use of copper conductor devices on semiconductor wafers, better systems capable of performing three dimensional defect analyses are more important than ever. This is because there are more defects that are buried and/or smaller, and in addition, chemical analysis is needed in many cases. Moreover, structural diagnostics solutions for defect characterization and failure analysis need to deliver more reliable results in less time, allowing designers and manufacturers to confidently analyze complex structural failures, understand the material composition, and source of defects, and increase yields.
For example, dual beam systems can be used to detect voids in copper interconnect trenches fabricated by a damascene process. In a typical damascene process, the underlying silicon oxide insulating layer of a substrate is patterned with open trenches where the copper conductor should be deposited. A thick coating of copper that significantly overfills the trenches is deposited on the insulator, and chemical-mechanical planarization is used to remove the copper to the level of the top of the insulating layer. Copper that is deposited down in the trenches of the insulating layer is not removed and becomes the patterned conductor. Any voids in the copper within the trenches can cause an open circuit defect. To evaluate the quality of the fill within the trenches, a dual beam system can be used to expose and image a cross section of the trench.
FIG. 1 shows a method for exposing a cross-section using a dual beam SEM/FIB system as known in the prior art. Typically, to analyze a feature within the sample 102, a focused ion beam (FIB) exposes a cross section, or face 108, perpendicular to the top of the surface 112 of the sample material having the hidden feature to be viewed. Because the SEM beam axis 106 is typically at an acute angle relative to the FIB beam axis 104, a portion of the sample in front of the face is preferably removed so that the SEM beam can have access to image the face. One problem with the prior art method is that a large number of cross-sections must typically be exposed along the length of the trench to form a set of samples of a sufficient size to properly characterize the trench.
For features that are deep relative to the opening that is being made by the FIB, the prior art method suffers from a reduced signal to noise ratio. The situation is analogous to shining a flashlight into a deep hole to try to form an image of the side of the hole. For example, a typical copper interconnect trench is 5-8 nanometers (nm) wide by 12 nanometers deep. Many of the electrons from the SEM remain in the trench and are not scattered back to the detector.
A common application for a dual beam system is in the field of biological sciences. For example, electron microscopy allows the observation of molecular mechanisms of diseases, the conformation of flexible protein structures and the behavior of individual viruses and proteins in their natural biological context. One technique employed with electron microscopy for analyzing biological materials, for example, is called “Slice-and-View.” This technique is typically performed with a dual beam SEM/FIB system.
In the slice and view technique, the FIB cuts and slices a sample with high precision to reveal its 3D internal structures or features. After obtaining an image of the face by the SEM, another layer of substrate at the face may be removed using the FIB, revealing a new, deeper face and thus a deeper cross-section of the feature. Since only the portion of the feature at the very surface of the face is visible to the SEM, sequential repetition of cutting and imaging, or slicing and viewing, provides the data needed to reconstruct the sliced sample into a 3D representation of the feature. The 3D representation is then used to analyze the sample feature.
The processing of a sample through a slice and view procedure can take a long time if a large section of the sample is processed. This is also true even if the feature of interest is relatively small in relation to the sample because the location of the feature is not typically known with sufficient accuracy to direct the beams of the FIB and SEM to the immediate region of the sample containing the feature. Therefore, a large section of the sample suspected of having the feature is processed in order to locate the feature. With a typical maximum field of view for the SEM being about 150 microns, slice milling and imaging an area this size can be a significant time investment, especially with high resolution settings on the SEM. Alternatively, many smaller portions of the area may be imaged, but doing so generates a vast amount of image data, and the resulting images are typically required to be stitched together to form a larger composite image. Such processes currently can last anywhere from a few hours to several days.
In prior art methods a relatively large section has been required to be processed with every iteration of the slice and view procedure because the shape or direction of the feature through the sample has not been accurately predicted. This problem is especially exacerbated with certain features that have long, winding shapes through the sample, such as is the case with blood vessels or nerves.